This invention relates to hydrocarbon fuel compositions, and more particularly relates to water-hydrocarbon fuel mixtures and systems for combusting such mixtures, including mixtures containing low-molecular weight hydrocarbons.
Federal clean air legislation has targeted fossil fuel emissions. This legislation has prompted engine manufacturers and fuel providers to look for solutions to be able to continue selling their products. Refiners must look at alternative formulations and/or blends to reduce emissions. Engine designers, on the other hand, must rethink the entire combustion process and how it is conducted from beginning to end.
Combustion engine manufacturers are increasingly employing the use of tighter tolerances on piston-wall machining to reduce engine oil burning. Additionally, combustion engine manufacturers are employing higher and higher combustion zone fuel injection pressures. Such higher pressures result in better spray penetration of fuel into the combustion zone as well as finer fuel droplet sizes, and the higher pressures permit smaller orifices at the injector tips while maintaining the same mass flow rate.
With newly developed fuel injectors operating at up to 30,000 psi, the fuel droplet size is reduced but the droplet dimensions are still in the 60 xcexcm-100 xcexcm range. A fuel droplet size reduction by a factor of two would necessarily be accompanied by an increase in droplet number by a factor of eight from a mass balance perspective. This is important because many small droplets improve the microscopic homogeneity and reduce particulate matter production. However, a reduction to 60 xcexcm-100 xcexcm droplet size at these extreme pressures achieves only about a factor of two in particle size reduction over the standard 3800 psi systems.
In general, droplet size of a fluid is primarily related to the viscosity or surface tension of the fluid. Therefore, any process that reduces these properties can potentially reduce the droplet size. A chemical approach to droplet size reduction has been proposed using surfactant technology. Viscosity-reducing or surface tension-reducing additives have been proposed, but high cost and other limitations have limited their efficacy.
The addition of heat to a hydrocarbon fuel reduces surface tension of the fuel. Thus, preheating of a fuel has some appeal from both an emissions and fuel economy perspective. However, when a fuel alone, without other components, is heated to temperatures above the critical temperature of the components of the fuel, e.g., about 250xc2x0 C., unwanted chemical reactions such as polymerization, oxidation, rapid decomposition, and other such reactions can occur, resulting in undesirable reformulation of the fuel into higher as well as lower molecular weight compounds. The fuel""s viscosity generally increases, due to these chemical reactions, at a rate that outpaces the drop in surface tension. As a result, a sticky, tarry residue can be produced. It has generally been found, therefore, that simple preheating of hydrocarbon fuels per se has limited use.
The addition of water to a heated fuel offers benefits, but water and hydrocarbons do not mix readily. The polar nature of water and the non-polar character of fuels favor phase separation into two unmixed pure liquids. Water does not exhibit an antibonding interaction with fuels. It simply has an overwhelmingly strong attraction for other water molecules that precludes bonding with hydrocarbon units. This phase separation property can be ameliorated by the addition of surfactants and cosurfactants, but such are generally expensive and can pose materials compatibility issues in use, among other issues.
The invention provides a low-molecular weight hydrocarbon and water mixture that overcomes the limitations of prior fuel compositions. In one embodiment of the invention, this mixture includes low-molecular weight hydrocarbon and between about 5% and about 70% water. The mixture is at a pressure that is below the critical pressure of the mixture. The mixture is all in the gaseous state and is at a temperature that is between about 28xc2x0 C. (50xc2x0 F.) below the boiling point temperature of water at the mixture pressure and about 450xc2x0 C. (842xc2x0 F.). The term xe2x80x9clow-molecular weight hydrocarbonxe2x80x9d as used herein includes methane, ethane, propane and other hydrocarbons which have a molecular weight below 60. Such hydrocarbons may be present as essentially pure compounds or as mixtures comprising two or more low-molecular weight hydrocarbon compounds. For example, pipeline natural gas is particularly useful herein and typically comprises more than about 90% methane, with the remainder typically being mainly ethane and propane.
This mixture provides a local environment of water molecules that tend to limit hydrocarbon polymerization and other undesirable side reactions and keep the hydrocarbon from precipitating from the mixture. This results in xe2x80x9ccleanerxe2x80x9d combustion of the mixture with reduced particulate matter and nitrous oxide emissions, compared with that which results from the combustion of otherwise comparable water-hydrocarbon mixtures at temperatures below about 28xc2x0 C. (50xc2x0 F.) less than the boiling point of water at the sub critical pressure employed. The present invention thereby provides for a faster, more complete combustion cycle that is characterized by lower emissions of carbon monoxide, unburned hydrocarbons, and volatile organic and polyaromatic hydrocarbons.
In embodiments provided by the invention, the water in the mixture is in a gaseous state, and low-molecular weight hydrocarbon constituents in the mixture are also preferably in a gaseous state. Expansion of such a mixture into the heated air of a cylinder of a combustion engine provides better mixing and combustion at a molecular level than conventional fuels and eliminates the time required for droplet evaporation.
In embodiments provided by the invention, the mixture preferably includes between about 5% and about 70% water with the remainder consisting essentially of low-molecular weight hydrocarbon, and more preferably between about 25% and about 60% water. The low-molecular weight hydrocarbon is preferably a hydrocarbon fuel, and can preferably be provided as, e.g., methane, ethane, or natural gas. The mixture can consist essentially of hydrocarbon fuel and water.
In further embodiments, the temperature of the mixture is maintained above about 14xc2x0 C. (25xc2x0 F.) less than the boiling point temperature of water at the mixture pressure. Where the hydrocarbon fuel is provided as methane, it can be preferable to maintain the mixture at a temperature below about 363xc2x0 C.
In a further aspect of the invention, there is provided a fuel system. The system includes a source of water and a source of low-molecular weight hydrocarbon fuel. At least one pump is provided, having an inlet connected to receive at least one of the water and the hydrocarbon fuel from the source of water and source of hydrocarbon fuel. The pump has at least one outlet for delivering at least one of a water stream and a hydrocarbon fuel stream to a volume to pressurize and transport the water and hydrocarbon fuel. As used herein, the term xe2x80x9cpumpxe2x80x9d is defined as a device which by mechanical action, increases the pressure of a flowing fluid, such as a liquid or a gas. Such devices for liquids include centrifugal pumps, gear pumps, reciprocating pumps, etc. Such devices for gases include centrifugal compressors (axial or radial), reciprocating compressors, lobed (Rootes) blowers, etc. At least one heater is provided in a connection to heat the water and hydrocarbon fuel. A mixer is connected to receive the water and hydrocarbon fuel to mix together the water and hydrocarbon fuel.
A water-hydrocarbon fuel mixture is provided in the volume. The mixture includes hydrocarbon fuel and between about 5% and about 70% water. The mixture is at a pressure that is below the critical pressure of the mixture and is at a temperature that is above about 28xc2x0 C. less than the boiling point temperature of water at the mixture pressure, and below about 450xc2x0 C. (842xc2x0 F.).
Preferably the volume is of sufficient structural integrity to contain a gaseous composition at a pressure above about 150 psi. Further, the pump is preferably of sufficient structural integrity to deliver at least one of a water stream and a hydrocarbon fuel stream at a pressure above about 2,000 psi. In embodiments of the invention, the fuel system includes an injector connected to an outlet of the volume and a combustion chamber for delivery of the water-hydrocarbon fuel mixture from the volume to the combustion chamber. The injector is preferably of sufficient structural integrity for delivery of the mixture to the combustion chamber under turbulent flow conditions.
In embodiments of the invention, the combustion chamber is a component of, e.g., a diesel engine, a spark ignition engine, a gas turbine engine, or a steam boiler.
In a further aspect of the invention, there is provided a method for forming and combusting a hydrocarbon-based fuel. In the method, a mixture is formed including a low-molecular weight hydrocarbon fuel and between about 5% and about 70% water. The mixture is pressurized to a pressure that is below the critical pressure of the mixture. The hydrocarbon fuel and the water of the mixture are heated to a temperature that is between about 28xc2x0 C. (50xc2x0 F.) less than the boiling point temperature of water at the mixture pressure and about 450xc2x0 C. (842xc2x0 F.). The mixture is then delivered to a combustion chamber and combusted.
In embodiments of the invention, the step of forming the mixture is carried out before the step of heating the mixture, and the step of heating the mixture is carried out by heating the mixture to a temperature above about 125xc2x0 C. (257xc2x0 F.). In other embodiments, the step of heating the hydrocarbon fuel and the water of the mixture is carried out before the step of forming the mixture, and is carried out by heating the water and hydrocarbon fuel separately. The method can be carried out employing, e.g., methane fuel, in which case the step of heating the mixture is preferably carried out by heating the mixture to a temperature that is below about 363xc2x0 C. (685xc2x0 F.).
The water-hydrocarbon mixture of the invention and the fuel system of the invention have applications for both mobile and fixed applications. For example, the mixture of the invention can be used in the combustor of a gas turbine or jet engine as used in aircraft, marine, automotive or stationary power generation applications.
Other applications, features, and advantages of the invention will be apparent from the following description and associated drawings, and from the claims.